Ready, set, go

The rapid switch from movement planning to execution is mediated by midbrain neurons that transmit information to cortex via the thalamus.

Declining sleep quality (including frequent night waking) is a common complaint associated with ageing; however, the mechanisms that underlie age-related changes in sleep patterns are unknown. In a study carried out in mice, Li et al. show that hyperexcitability of a specific population of hypothalamic neurons drives ageing-related sleep fragmentation.
Specific neuronal circuits control sleep onset and the transi tion from sleep to wakefulness. It has been hypothesized that broken sleep in ageing animals results from increased activity within arousal-promoting circuits. To explore this idea, Li et al. focused their attention on a key component of these circuits -a population of neurons in the lateral hypothalamus that release the neuropeptide hypocretin (HCRT) (also known as orexin) and are known to have a role in initiating and maintaining wakefulness.
Using a calcium indicator expressed specifically in lateral hypothalamic HCRT neurons, the authors compared the intrinsic activity of this neuronal population in aged mice (18-22 months old) and young mice (3-5 months old), while simultaneously assessing the sleep-wake patterns of the animals. They found that calcium transients associated with wakefulness occur more frequently in aged mice, corresponding to an increased frequency of waking and a shorter duration of each sleep-wake cycle. Optogenetic activation of the HCRT neurons during sleep triggered wakefulness in all mice; however, the transition to waking occurred faster in aged mice than in young mice and the animals remained awake longer. These effects were observed despite a reduction in the total number of lateral hypothalamic HCRT neurons in aged mice, suggesting that the S L E E P Too excited to sleep surviving HCRT neurons in aged mice may be hyperexcitable, reducing the threshold of stimulation needed to trigger sleep-wake transitions.
To investigate this possibility, the authors obtained whole-cell patchclamp recordings from genetically identified HCRT neurons in brain slices from young and aged mice. They discovered that HCRT neurons exhibit a more-depolarized resting membrane potential (RMP) in the aged mice than in the young mice, reducing the depolarization required for an action potential to be triggered. Correspondingly, the aged-mouse neurons exhibited more spontaneous activity and were more responsive to external stimulation than were the neurons from young mice.
Next, the authors considered how changes in the molecular properties of aged HCRT neurons might underlie their hyperexcitability. Voltage-gated potassium channels composed of KCNQ2 and KCNQ3 subunits mediate the M current (I M ) that repolarizes a neuron's RMP after firing. The authors observed reduced voluntary movements are often planned before execution and released by spec ific sensory events; for example, the sound of a gun going off at the beginning of a track race ('go cue'). Neuronal dynamics in motor-related brain areas switch dramatically at around the time of movement onset. Slowly varying 'preparatory activity', which mediates motor planning, changes into a distinct activity pattern that has roles in move ment initiation. What are the neuronal mechanisms underlying the transition between them? Inagaki et al. identify a multiregion pathway through which information related to the go cue flows to reorganize population neuro nal activity and release planned movements.
To study the neuronal dynamics and mechanisms underlying movement planning and execution, the authors used a delayed directional licking task. mice were instructed to lick that that the population activity patterns in Alm changed rapidly before and during movement initiation. Consistent with previous research, Alm showed preparatory activity predicting lick direction before the go cue. After the go cue, preparatory activity attenuated rapidly, and movement-specific activity and movement-type nonselective activity (referred to as 'D go ') emerged.
The authors sought to dissociate the function of movement-selective activity

Ready, set, go
in one of two directions, but only after an auditory go cue, which was delivered one second after the instruction. The authors focused their attention on anterior lateral motor cortex (Alm), which is necessary for motor planning and execution of directional licking, and a part of the thalamus that has reciprocal direct connections with Alm (thal Alm ) and also relays input to Alm from subcortical structures.
large-scale electrophysiological recordings during the lick task revealed a midbrainthalamusmotor cortex circuit signals a contextual cue to reor ganize ALM dynamics and release planned movement surviving HCRT neu rons in aged mice may be hyperexcitable, reducing the threshold of stimulation needed to trigger sleep-wake transitions KCNQ2 expression and a lower basal I M in aged HCRT neurons -changes that might drive hyperexcitability. Indeed, perfusing brain slices with a KCNQ2/3 blocker depolarized the RMP and increased the firing frequency of young HCRT neurons, whereas a KCNQ2/3 activator hyperpolarized the RMP and reduced firing frequency in aged HCRT neurons.
To validate the link between the impaired I M and disrupted sleep patterns in aged mice, the authors used CRISPR-mediated gene editing to disrupt Kcnq2 and/or Kcnq3 in HCRT neurons in young mice. Over the following 8 weeks, the mice exhibited fragmented sleep patterns similar to those of aged mice, as well as a depolarized RMP and increased spontaneous activity in HCRT neurons. Similar effects were seen upon administration of a KCNQ2/3-selective blocker to young mice, whereas a KCNQ2/3 activator increased sleep stability in aged mice.
This work provides support for the hypothesis that increased intr insic activity in the circuits that drive sleep-wake transitions contributes to age-related sleep disruption, and suggests possible avenues for the therapeutic restoration of healthy sleep patterns in aged populations.

Katherine Whalley
the movement-specific activity emerged in Alm, and the mouse licked in the correct direction -that is, it released the planned movement. In addition, optogenetic perturbation of PPN/MRN th neuronal activity following the go cue resulted in loss of the go cue response in Alm. Altogether, these results show that a midbrain-thalamus-motor cortex circuit signals a contextual cue to reorganize Alm dynamics and release planned movement.
By combining cell-type-specific optogenetic manipulations of neural activity and large-scale elec trophysiology, this research causally links neuronal dynamics with a specific computational role in behaviour. many behaviours are composed of sequential steps, with switching of computations in between. A similar approach could be applied to study other sequenced behaviour.
Sian Lewis and D go . Silencing medulla-projecting pyramidal-tract ALM cells (PT lower ), the major descending neurons of Alm, prevented mice from licking in response to the go cue, suggesting an important role in movement initiation. Silencing PT lower cells also attenuated movement-selective activity, but not D go , implying that D go is not sufficient to trigger movement by itself. Instead, D go causes the transition from preparatory activity to movementspecific activity, which presumably functions as a motor command. The authors mapped the pathway linking the auditory go cue and the D go signal in Alm. They investigated the latency of the go cue across brain regions. They found that D go emerges first in neurons in the pedunculopontine nucleus (PPN) and midbrain reticular nucleus (MRN) that project to thal Alm (PPN/MRN th ), followed by thal Alm and then Alm. The authors transiently activated PPN/MRN Th neurons to mimic the go cue and found that this produced D go in Alm. When the amplitude of D go was sufficiently large, preparatory activity collapsed, R e s e a R c h h i g h l i g h t s The RNA-binding protein TAR DNAbinding protein 43 (TDP-43) acts in the cell nucleus, where it represses the inclusion of cryptic exons in RNA during splicing. In amyotrophic lateral sclerosis (AlS) and frontotemporal lobar degeneration (FTLD), TDP-43 is depleted from the cell nucleus and instead accumulates in the cytoplasm. However, it is not clear which cryptic exons might contribute to disease in cells with Ma et al. and Brown et al. show that TDP-43 depletion promotes the inclusion of a cryptic exon in transcripts of the UNC13A gene, reducing the expression of uNC13A (a protein that is crucial for synaptic-vesicle fusion).
Ma et al. analysed RNA-sequencing data from brain tissue of individuals who died with AlS-FTlD, and found that UNC13A was one of the genes pathologically spliced in nuclei lacking, but not those containing, TDP-43. Brown et al. performed RNA sequencing on induced pluripotent stem cellderived neurons depleted of TDP-43. In these cells, but not in cells containing TDP-43, UNC13A included a cryptic exon in the intron between exons 20 and 21. Both groups found that inclusion of the UNC13A cryptic exon triggered reductions in the levels of UNC13A mRNA and UNC13A protein.
The groups analysed brain samples from people who had AlS, FTlD or AlS-FTlD. Notably, the UNC13A cryptic exon was observed only in individuals who had forms of these disorders associated with TDP-43 pathology, and not in individuals with other proteinopathies. moreover, the cryptic exon tended to be observed only in the nuclei of cells that showed a depletion of TDP-43. Ma et al. and Brown et al. both showed that TDP-43 binds directly to the intron containing the cryptic exon. Both groups focused on two single-nucleotide polymorphisms (SNPs) previously associated with increased risk for AlS-FTlD, which are both in the cryptic exon-containing intron of UNC13A. Both groups showed using minigene constructs that these risk SNPs increase the inclusion of the cryptic exon in cells lacking TDP-43, suggesting that these SNPs impair the ability of TDP-43 to facilitate the proper splicing of UNC13A. In line with this, individuals who had these SNPs and TDP-43 pathology showed more cryptic exon inclusion in UNC13A transcripts than did individuals with TDP-43 pathology but no risk SNPs.
Together, these studies identify a mechanism that links TDP-43 pathology with risk SNPs for ALS and FTLD.